Complications from arterial diseases such as myocardial infarctions (MI) and cerebral vascular accidents (CVA) cause a majority of the deaths in the western world. The survival of a patient is highly dependent upon the early diagnosis and treatment of the stenotic lesions which characterize these diseases. Dye contrast X-Ray angiography has long provided an invasive means of imaging the arteries. Drawbacks of this procedure are its invasive nature and the potentially harmful exposure of the patient to ionizing radiation. Magnetic resonance angiography (MRA) was initially thought to be the answer to these problems. It is a non-invasive procedure, and the evidence to date is that the high magnetic fields pose no danger to human cells. However, all MRA imaging techniques in use today fail in the critical area of arterial disease detection. Massive signal loss occurs when imaging blood vessels having stenotic flow which is often encountered with arterial diseases, causing an overestimation of the extent of stenosis or even a misrepresenting of a diagnosis of total occlusion.
Magnetic resonance imaging (MRI) has the ability to isolate and differentiate moving protons from static protons. Therefore, MRI can identify fluid motion as a bulk of moving protons. It is this feature of MRI which makes possible the creation of MR angiograms, or pictures of arteries. Magnetic resonance (MR) angiograms are typically described as either being "black flow" or "white flow," The term black flow implies that the flowing fluid returns a low echo signal relative to the background signal level. The term white flow implies that the flowing fluid returns a high signal relative to the background. To obtain a greater understanding of this concept, see Richard Underwood & David Firmin, Magnetic Resonance of the Cardiovascular System, pp. 107-130 (1992).
Magnetic resonance angiograms provide useful images for many flow conditions such as laminar flow. However, MR white flow angiograms provide poor images of turbulent flow because in regions of highly turbulent flow the image appears dark, rather than light, making the geometry of the vessel indistinguishable from the dark background tissue. In particular, diseased arteries usually create a high level of acceleration and other abnormal physiologic flow patterns which can result in a whole length (10 diameter or more) of the artery appearing dark, creating the illusion of a blocked artery. This dark region is termed "signal loss." MR black flow angiograms experience similar problems in diseased vessels. Often arterial constrictions will produce a region of slowly moving fluid distal to the occlusion. This region appears white on a black flow angiogram, whereas the rest of the vessel will appear dark. In the context of this document, this error in MR angiograms is referred to as "signal loss." The major point is that signal loss occurs when the MR image loses its ability to differentiate between a moving fluid and the surrounding material.
Turbulent signal loss has been documented in a wide variety of flow geometries such as aortic coarctations (Simpson et al.,"Cine Magnetic Resonance Imaging for Evaluation of Anatomy and Flow Relations in Infants and Children with Coarctation of the Aorta," Circulation, v. 78, pp. 142-448 (1988); Mirowitz et al., "Pseudocoarctation of the Aorta: Pitfall on Cine MR Imaging," J Comput Assist Tomography, v. 14, pp. 753-754 (1990)), orifice flow (Evans et al., "Effects of Turbulence of Signal Intensity in Gradient Echo Images," Invest Radiology, v. 23, pp. 512-518 (1988)), and cosine shaped stenosis (Krug et al., "MR imaging of Poststenotic Flow Phenomena: Experimental Studies," JMRI, v. 1, pp. 585-591 (1991); Oshinski, "MRI of Stenotic Flows," PhD Thesis, Georgia Institute of Technology (1993)). A widely recognized means for decreasing the area of turbulent signal loss is reduction of the image echo time or the applied gradient duration. In this regard, refer to the following articles: Schmalbrock et al., "Volume MR Angiography: Methods to Achieve Very Short Echo Times," Radiology, v. 175, pp. 861-865 (1990); Kilner et at., "Valve and Great Vessel Stenosis: Assessment with MR Jet Velocity Mapping," Radiology, v. 178, pp. 229-235 (1991); Urchuk et al., "Mechanisms of Flow-induced Signal Loss in MR Angiography," JMRI, v. 2, pp. 453-462 (1992); Gatenby et al., "An Investigation using Partial Echo Techniques of Post-stenotic Signal Loss." 11th Annual SMRM Conference Abstracts, p. 364 (1991); and Gatenby et al., "Mechanisms of Signal Loss in MR Imaging of Stenoses," 12th Annual SMRM Conference Abstracts, p.2814 (1992). However, it is also widely recognized that conventional magnetic resonance imaging hardware limits the minimum echo time, and therefore, these systems cannot provide a small enough echo time to image diseased vessels in the human anatomy.
The theoretical explanations for signal loss vary with the gradient sequence used. One hypothesis assuming only velocity encoding is described in Nalcioglu et al., "Application of MRI in Fluid Mechanics of Turbulent Flow," 7th Annual SMRM Abstracts, p. 418 (1987). A second hypothesis with a partial gradient echo sequence is described in Gatenby et al., "Measurement of Turbulent Intensity using Partial Echo Techniques," 12th Annual SMRM Conference Abstracts, p.2815 (1992). Both suggest that the fluctuating component of velocity in turbulent flow causes an exponential decline in signal intensity. The same phenomena is said to occur on the even echoes of a multiple echo-spin echo sequence. See De Gennes, "Theory of Spin Echoes in a Turbulent Fluid," Physics Letters A, v. 29, pp. 20-21 (1969); Fukuda et al. , "A Pulsed NMR Study on the Flow of Fluid," J. Physics Society of Japan, v. 47, pp. 1999-2006 (1979); and Kawabe et al., "A Note of the Signal Intensity of the Second Spin Echo in Carr-Purcell- Meiboom-Gill Sequence of Turbulent Flows," J. Physics Society of Japan. v. 52, pp. 2308-2311 (1983). Others argue that signal loss using spin-echo imaging, as discussed in Kuethe et al., "Fluid Shear and Spin-Echo Images," MRM, v. 10, pp. 57-70 (1989), or echo-planer imaging, as discussed in Kose, "Visualization of Shear Distribution in Turbulent Fluids Using Echo-Planar Imaging," 11th Annual SMRM Conference Abstracts, p. 362 (1991), can be caused by fluid shear stress.
In summary, there is no consistent explanation and solution for the signal loss seen in MR angiograms of diseased arteries. Likewise, no methods have been published which consistently eliminate the problem of signal loss in MR angiograms.